Defecation depends on
a. distention of the rectal wall
b. sensory impulses to the sacral spinal cord
c. motor impulses back to the colon
d. all of the above must happen for defecation to occur

the correct answer is d. all of the above must happen for defecation to occur.

Explanation:

In response to the distention of the rectal wall, the receptors send sensory nerve impulses to the sacral spinal cord. Motor impulses from the cord travel along parasympathetic nerves back to the descending colon, sigmoid colon, rectum, and anus. The resulting contraction of the longitudinal rectal muscles shortens the rectum, thereby increasing the pressure within it. This then opens the internal sphincter.

measuring light intensity

light meters can be used to measure light intensity. the meter is held at the soil surface and pointed in the direction of the maximum light intensity, and then the meter is read.

errors can be made when measuring light intensity by accidentally shading the light meter. the reliability of the results can be checked by taking many samples.